"W" rated oils mean better flow characteristics below 32 degrees F. and improves lubrication to very cold parts. Take 10W-30 oil as an example:
When the oil is cold it acts as a 10-weight oil, it's thin and flows quickly. Has is warms up its viscosity gradually transforms to a 30-weight oil.
There is a lot of chemical miracles involved in developing modern engine oils. With OHV engines especially, the most critical time for top end lubrication is at startup.
There are lots of good articles and videos online from Bob the Oil Guy and Lake Speed Jr. on the subject
